This invention relates generally to piezoelectric materials, and more particularly to PZT-polymer 1-3-0 phase-connected composites for hydrophone applications.
Hydrophone devices use piezoelectric materials to act as passive listening devices for low frequency acoustic waves, which produce an essentially hydrostatic pressure on the devices. The sensitivity of a hydrophone is determined by the voltage that is produced from this hydrostatic pressure. The hydrostatic piezoelectric voltage coefficient (g.sub.h) relates the electric field (voltage/thickness) of a piezoelectric material to the applied hydrostatic stress, and is therefore a useful parameter for evaluating a material for use in a hydrophone. The hydrostatic piezoelectric charge coefficient (d.sub.h) relates to the polarization (charge/area) produced from an applied hydrostatic stress. The relationship between g.sub.h and d.sub.h is given by: EQU g.sub.h =d.sub.h /K.sub.33 .epsilon..sub.0 Eq. ( 1)
where K.sub.33 is the dielectric constant in the x.sub.3 direction (the poling direction), and .epsilon..sub.0 is the permittivity of free space. A large charge coefficient d.sub.h and low dielectric constant K.sub.33 are desired so the hydrophone material may have a large voltage coefficient g.sub.h and thus have high sensitivity. The product of d.sub.h and g.sub.h is often used as the figure of merit of a material for use in hydrophone applications.
In addition to having large d.sub.h and g.sub.h coefficients, there are other desirable hydrophone material characteristics. The piezoelectric element within the device should be acoustically impedance-matched to water, should have high compliance for resistance to mechanical shock from pressure fluctuations, and should be flexible if the device is to be mounted on the hull of a ship.
Lead zirconate titanate (PZT) has traditionally been used for hydrophone devices, but it does not have many of the desired qualities. Piezoelectric composites of PZT and a polymer have been fabricated to combine the desirable properties of each phase. The PZT supplies the piezoelectric activity of the composite, while the polymer lowers the dielectric constant K.sub.33 and density, and adds flexibility. Through the proper selection of the connectivity and properties of these phases, the d.sub.h coefficient can be enhanced. By replacing most of the PZT (high K.sub.33) with a polymer (low K.sub.33), the dielectric constant can be significantly reduced, resulting in an enhanced g.sub.h coefficient.
One type of connectivity pattern that has been particularly successful is the 1-3 composite with PZT rods aligned in the poling direction (x.sub.3 direction) held together by a polymer matrix. In the notation 1-3, one (1) refers to the one-dimensionally connected PZT phase and the three (3) refers to the three-dimensionally connected polymer phase. The stiffer PZT rods support most of an applied stress in their direction of alignment, the x.sub.3 direction, because of the parallel connection with the more compliant polymer phase. Hence, the d.sub.h coefficient for a 1-3 composite can be increased compared to its low value for single-phase PZT.
Porosity has been added to the matrix to allow it to be compressible. 1-3-0 connectivity notation has traditionally been used to describe a 1-3 composite with a porous polymer matrix (i.e. polymer in the form of a foam). The 0 refers to the polymer porosity, which is not connected in any orthogonal direction through the composite. With the addition of porosity, the internal stresses are decreased, which enhances the d.sub.h coefficient. However, at high pressures, the small voids collapse and the enhanced effect is lost.